Darn it, I was afraid that wouldn't work. ☹️ Your idea to transfer is a good one, unfortunately they only made it so that you can take the multiplayer to single player mode, not the other way around.
You could try to salvage the region you've built by transferring it to single user mode, but I fear that problem cities won't transfer well. I've never personally done it with a city that is in the middle of rolling back, but I've seen reports where people who have tried it end up with a city that is marked like it's another person's city. So, you can visit, but not play that city.
As far as the recovery, I believe if you navigate to the file itself - don't use shortcuts from the desktop or menu - it is an exe file, so shouldn't take you to any web page. You may have to turn on the option to view file extensions and hidden files & folders. In file explorer, select View > Options > Change folder and search options. In the View tab, choose Options, then under Hidden Files & Folders, choose "Show hidden files, folders and drives" and Un-Check "Hide extensions for known file types". Then go to C:\Program Files (x86)\Origin Games\SimCity\SimCityRecovery, right-click SimCityRecovery.exe, click "Run As Administrator" and accept the prompts to get it started.
I sent a PM to a Community Manager who may be able to help, although their access to server-side kinds of things are very limited, perhaps we can get some other ideas. I really hope we can get your cities up and running again. Your only options may be to start another region (more fun and practice?), or run the region without playing the affected cities. Not great choices, I know. I wish I could run down to the server room and defrag it for you, but they seem to frown on non-employees randomly showing up and messing with stuff :D